My solution to a challenge from https://codefights.com

reverseParentheses.js

/*
Samuel Plumppu - 2018-02-27

You have a string s that consists of English letters, punctuation marks, whitespace characters, and brackets. It is guaranteed that the parentheses in s form a regular bracket sequence.

Your task is to reverse the strings contained in each pair of matching parentheses, starting from the innermost pair. The results string should not contain any parentheses.

Example

For string s = "a(bc)de", the output should be
reverseParentheses(s) = "acbde".
*/


function reverseParentheses (s) {
    let result = s
    while (result.includes('(')) {
        result = reverseParenthesis(result)
    }
    return result
}

// This regex matches a complete parenthesis that has no other parentheses inside of it.
const innermostParenthesis = /\([^(]*?\)/

// The negated set "[^(]" makes sure that we only capture the innermost parenthesis.
// Likewise, the lazy quantifier "?" makes sure that it stops when the innermost parenthesis ends.
    
function reverseParenthesis (s) {
    // Replace the deepest parenthesis with its reversed content.
    // 1. Split into chars.
    // 2. Slice to remove first and last chars (the parenthesis).
    // 3. Reverse and join to a string.
    return s.replace(innermostParenthesis, content => (
        [...content].slice(1, -1)
            .reverse().join('')
    ))
}